They're the bomb diggity.
True rotary valve engine. Super fast and reliable. I'd buy one in a heart beat if I could find one for reasonable.
What you should do is mess with your intake timing (tune the rotary valve), put a chamber on the stock header, jet it and port the stock cylinder. They're cast iron, so no stupid fucking plating to worry about.
You can flow plenty enough air through the stock inlet and carb.